1. Field of the Invention
The present invention relates to a control system for an internal combustion engine that performs catalyst temperature rise accelerating control for accelerating the rise in temperature of a catalyst used for exhaust gas purification, and particularly to a control system that performs the control for the case in which the vehicle driven by the internal combustion engine starts to creep during execution of the catalyst temperature rise accelerating control.
2. Description of the Prior Art
The catalyst provided in an exhaust system of an internal combustion engine for purifying exhaust gases is inactive at low temperatures. Consequently, the catalyst does not purify the exhaust gases at low temperatures. Therefore, it is desirable to make the catalyst active early by rapidly increasing the temperature thereof immediately after start-up of the engine. In this regard, there is known a catalyst temperature rise accelerating control (disclosed in Japanese Patent Laid-open No. Hei 10-299631) in which immediately after start-up of an internal combustion engine, the intake air amount supplied to the engine is made larger than the amount for normal idling operation and also the ignition timing is feedback-controlled in the retard direction so that the engine rotational speed coincides with a target rotational speed, to thereby accelerate the temperature rise of the catalyst while keeping the engine rotational speed at the target rotational speed. According to this control, the temperature rise of the catalyst can be accelerated because the fuel supply amount becomes larger with the increasing intake air amount and thereby the heat generation amount becomes larger than that in the normal idling operation.
The catalyst temperature rise accelerating control is ended after the vehicle driven by the internal combustion engine starts running. However, if the control is immediately shifted to normal control, a torque shock occurs because of a rapid advance of the ignition timing as well as a rapid reduction in intake air amount. To cope with such an inconvenience, a transient control for gradually reducing the intake air amount while continuing the feedback control of the ignition timing is generally executed. With this transient control, the ignition timing is gradually changed in the advance direction, and the feedback control is ended at a time when the ignition timing becomes closer to a normal control value.
However, if the above-described prior art control method is applied to an engine of a vehicle having an automatic transmission, there may be a case in which a torque shock occurs due to a rapid change in the ignition timing when the catalyst temperature rise acceleration control is shifted to the normal control.
To be more specific, when the automatic transmission is in an in-gear state (a shift position is in a drive range or reverse range) and braking is released, the vehicle starts creeping, that is, running at a low speed. When the vehicle speed reaches a predetermined vehicle speed after the start of creeping, the catalyst temperature rise accelerating control is ended, and is shifted to a transient control for gradually reducing the intake air amount. At this time, since the engine rotational speed is increased with an increase in vehicle speed, the ignition timing is controlled to be retarded more than the retard amount during execution of the catalyst temperature rise accelerating control in order to keep the engine rotational speed at a constant value, which results in the condition that the ignition timing sticks to a value in the vicinity of the limit value on the retard side. As a result, there arises a problem that at the time when the vehicle speed is further increased and the feedback control of the ignition timing is ended, that is, when the normal control is started, a changed amount of the ignition timing in the advance direction becomes very large, resulting in a torque shock.
An object of the present invention is to provide a control system for an internal combustion engine, which is capable of preventing the occurrence of a torque shock, thereby maintaining good drivability in the case where a vehicle having an automatic transmission starts creeping during execution of catalyst temperature rise accelerating control.
To achieve the above object, according to a first aspect of the present invention, there is provided a control system for an internal combustion engine having an exhaust system provided with a catalyst. The control system comprises intake air amount control means for controlling the intake air amount of the engine; ignition timing control means for controlling the ignition timing of the engine; and catalyst temperature rise accelerating means for increasing, after starting of the engine, the intake air amount and retarding the ignition timing according to the engine rotational speed. The catalyst temperature rise accelerating means starts reduction of the intake air amount when the vehicle speed of the vehicle driven by the engine reaches a first predetermined vehicle speed or higher during operation of the catalyst temperature rise accelerating means, ends retard control of the ignition timing according to the engine rotational speed when the vehicle speed reaches a second predetermined vehicle speed higher than the first predetermined vehicle speed, and changes the retard limit value of the ignition timing according to the vehicle speed when the vehicle speed is between the first predetermined vehicle speed and the second predetermined vehicle speed.
With this configuration, the reduction in intake air amount is started when the vehicle speed reaches the first predetermined vehicle speed or higher during operation of the catalyst temperature rise accelerating means, and the retard control of the ignition timing according to the engine rotational speed is ended when the vehicle speed reaches the second predetermined vehicle speed higher than the first predetermined vehicle speed. Further, the retard limit value of the ignition timing is changed according to the vehicle speed when the vehicle speed is between the first predetermined vehicle speed and the second predetermined vehicle speed. Accordingly, by setting the retard limit value of the ignition timing at an optimum value according to the increase in vehicle speed, it is possible to prevent the occurrence of a torque shock and hence to maintain good drivablity in the case where a vehicle having an automatic transmission, starts creeping during execution of catalyst temperature rise control.
The catalyst temperature rise accelerating means preferably advances the retard limit value a greater amount as the vehicle speed becomes higher when the vehicle speed is between the first predetermined vehicle speed and the second predetermined vehicle speed.
With this configuration, since the retard limit value of the ignition timing is advanced a greater amount as the vehicle speed becomes higher when the vehicle speed is between the first predetermined vehicle speed and the second predetermined vehicle speed, it is possible to reduce the amount of change in the ignition timing at the time of ending the ignition timing control according to the engine rotational speed, to thereby prevent occurrence of torque shock.
The operation of the catalyst temperature rise accelerating means is preferably started when the engine rotational speed is equal to or higher than a predetermined lower limit rotational speed and the engine is in an idling condition.
The control system preferably further includes engine coolant temperature detecting means for detecting the engine coolant temperature, and the operating period of the catalyst temperature rise accelerating means is set according to the engine coolant temperature at starting of the engine.
The catalyst temperature rise accelerating means preferably sets the retard amount of the ignition timing so that the engine rotational speed coincides with the target rotational speed.
According to a second aspect of the present invention, there is provided a control system for an internal combustion engine having an exhaust system provided with a catalyst. The control system comprises intake air amount control means for controlling the intake air amount of the engine; ignition timing control means for controlling the ignition timing of the engine; and catalyst temperature rise accelerating means for increasing, after starting of the engine, the intake air amount and retarding of the ignition timing according to the engine rotational speed. The catalyst temperature rise accelerating means starts a transient control shifted to a normal control when the speed of the vehicle driven by the engine reaches a predetermined vehicle speed or higher during operation of the catalyst temperature rise accelerating means, and gradually reduces the intake air amount and sets the ignition timing according to the vehicle speed during the transient control.
With this configuration, the transient control shifted to the normal control is started when a vehicle speed becomes the predetermined vehicle speed or higher during operation of the catalyst temperature rise accelerating means. During the transient control, the intake air amount is gradually reduced and the ignition timing is set according to the vehicle speed. Accordingly, by setting the ignition timing at an optimum value according to the increase in vehicle speed, it is possible to prevent occurrence of a torque shock and hence to maintain good drivablity in the case where a vehicle having an automatic transmission starts creeping during execution of catalyst temperature rise control.
The catalyst temperature rise accelerating means preferably advances the ignition timing a greater amount as the vehicle speed becomes higher during the transient control.
Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings.